§H.12 step 13: blk_meta_t flags bit constants

BLK_FLAG_CLAIMED/BLK_FLAG_MIGRATING/BLK_FLAG_STALE (bits 0/1/2), matching
the decided §F.4/§H.6 layout. Orthogonal bits, not a mutually-exclusive
enum.

Verified 3-arch boot to ok> (amd64/aarch64/riscv64).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

FABRIC-3.md

@@ -4059,7 +4059,10 @@ work, not new invention.
   comment to point at the assert instead of repeating the wrong number. Did not touch
   `BLK_META_PER_BLOCK` itself — out of scope, unused, not what this step asked for.
   Verified 3-arch boot to `ok>` (amd64/aarch64/riscv64).
-- [ ] **13.** Define the three `flags` bit constants (`CLAIMED`=0/`MIGRATING`=1/`STALE`=2).
+- [x] **13. DONE 2026-09-03.** `BLK_FLAG_CLAIMED`/`BLK_FLAG_MIGRATING`/`BLK_FLAG_STALE`
+  (bits 0/1/2, `1ull << n`) defined in `block_subsystem.h` right above `blk_meta_t`, `flags`
+  field comment updated to point at them. Verified 3-arch boot to `ok>`
+  (amd64/aarch64/riscv64).
 - [ ] **14.** Add C get/set accessors for the new fields in `block_subsystem.c`.
 - [ ] **15.** Add FORTH wrappers (`BLK-ACL-ALLOW@`/`!`, `BLK-ACL-TTL@`/`!`, `BLK-OWNER@`).
 - [ ] **16.** Add a new policy capsule (mirroring `ACL.4th`) with a real fast-deny check on